1. Field of the Invention
This invention relates to the field of theft security or anti-theft systems for merchandise. More particularly, this invention relates to an anti-theft security system that comprises a base station that generates an electromagnetic field that is in contact with one or more security labels attached to merchandise. In addition, the invention concerns a process for the automatic detection and identification of a merchandise security label by a base station so that when merchandise with the security label enters the electromagnetic field an alarm will sound alerting others to the theft of the merchandise.
2. The Prior Art
Electric security systems are employed in department stores for the purpose of better protecting high-priced merchandise against theft. A base station is located for this purpose at the exit of these department stores. These base stations generate an electromagnetic field covering the area of the exit. Merchandise security labels are attached to the goods to be protected. After a piece of merchandise has been properly paid for, the security label is either removed or destroyed at the cash register. When a person with the paid merchandise passes through the electromagnetic field located at the exit of the department store, the base station does not react. However, if a person wishes to pass the electromagnetic field with merchandise still having a functioning security label attached to it, then this security label is detected by the base station, whereupon an alarm is triggered.
Security labels that generate an electromagnetic response signal from the energy absorption of a signal sent from a base station are known in the art. Such an anti-theft security system is described, for example in EP 0 663 657 A1. A system operating with energy absorption employs an oscillating circuit in the security label, with this circuit being formed by a coil and a capacitator. This oscillating circuit generates a second magnetic alternating field when entering into the electromagnetic field of the base station. These second magnetic alternating fields can be detected via an antenna. Such systems usually operate in the HF-range from 1 to 2 MHz.
Other systems are known in which a ferromagnetic material is used as the security label, and where the base station operates in the manner of a metal detection device. With this device, an electromagnetic field is generated by the base station, and influenced by interferences caused by the ferromagnetic element, whereupon such an element is detectable.
Even though a great number of thefts can be prevented with this known anti-theft security system, it is possible to "outwit" such a system by screening the security label from the electromagnetic field generated by the base station before passing through the electromagnetic field. A piece of merchandise can be screened by wrapping the entire piece with the security label in aluminum foil or other metallic foil materials. If larger pieces of merchandise are involved, an aluminum suitcase could be used to outwit the system. In this case, if the electromagnetic field generated by the base station is not influenced, no response signal is received by the base station, and thus, no alarm is triggered.
These known systems also pose problems in situations where several of these security labels enter in the base stations's field of detection at once. In this case, the base station, which is in the alarm position, is then blocked by the first security label, and subsequent security labels entering the range of detection of the base station can then pass the latter unnoticed. A security label hidden within the range of such a base station can permanently put the entire installation out of operation.
Based on the state of the art discussed above the invention is therefore based on overcoming prior art by providing an anti-theft security system that is more reliable and less sensitive to manipulation.